1. Field of the Invention
The present invention relates to the field of power supply circuits for electrical and electronic equipment.
2. Related Art
Most electrical and electronic appliances today provide a power switch allowing the user of the appliance to selectively turn the appliance on and off. When a power switch of an appliance is at its `on` position, current flows through a completed circuit and a power supply unit supplies power to the appliance for its operation. Conversely, when a switch is at its `off` position, power supply to the appliance is cut off and the appliance is effectively turned off. Depending on the design and application of a power supply circuit, different kinds of power switches with varying specifications are used.
Moreover, in a power supply circuit with an alternating current (AC) power input, a transformer is commonly used for voltage conversion. The power input is located in the primary circuit of the transformer and supplies the transformer with an input signal of a certain voltage. Depending on its specifications and settings, the transformer provides the secondary circuit with an output voltage that is proportional to, and higher (stepped up) or lower (stepped down) than, the input voltage in the primary circuit. Thus, by utilizing a transformer of proper specifications and settings, an output voltage suitable for the operation of an appliance coupled to the secondary circuit can be selected.
To implement power switching in a circuit as described above, a switch can be installed in the primary circuit to selectively supply and cut off the input AC voltage to the transformer, thereby controlling the output voltage and indirectly turning the appliance in the secondary circuit on and off. Alternatively, a switch can be installed in the secondary circuit to directly control current flow in the secondary circuit and thus switch the appliance on and off.
FIGS. 1A and 1B illustrate a common configuration of a power supply circuit 5 with an alternating current (AC) power input 10. In the primary circuit 6a of this prior art configuration 5, a switch 12 is coupled between the AC power input 10 and a power supply controller 15, which is in turn coupled to a transformer 17. The switch 12 can either be a double-cut switch 12, as illustrated in FIG. 1A, or a one-side switch 12, as illustrated in FIG. 1B. When the switch 12 is opened, the connection between the AC power input 10 and the power supply controller 15 is broken and thus the power supply controller 15 no longer drives the transformer 17 to provide power to the secondary output 18 in the secondary circuit 6b. Consequently, any load connected to the secondary output 18 is effectively turned off.
However, in this prior art configuration 5, a very large transient inrush current flows through the switch 12 when the switch 12 is closed initially. As an example, for a 17-inch cathode ray tube (CRT) display monitor with an AC power input of 240 volts, the magnitude of the inrush current can range from 30 amperes to 100 amperes when the switch 12 is first closed, with the actual magnitude of the inrush current dependent, in particular, on the input power and the total input circuit impedance. In order to withstand this enormous inrush current, the switch 12 in this prior art configuration 5 must meet strict mechanical and flammability requirements. As a result, the cost of the switch 12 is very high to meet these current requirements. It would be advantageous to reduce the cost of the power supply by reducing the electronic component costs.
Another disadvantage of this prior art circuit 5 that arises out of the strict mechanical and flammability requirements of the switch 12 is that the switch 12 inevitably has a larger size than one with less severe specifications. The larger switch 12 takes up much more room of the circuit board which houses the power supply circuitry than a smaller conventional switch. The higher space consumption means that the resulting circuitry is more cumbersome, and that the placement of other circuit components is less flexible. Moreover, in order to reduce the magnitude of the inrush current through the switch 12, additional circuit elements such as an impedance element 11 coupled between the AC power input 10 and the switch 12 has to be included to raise the total input impedance of the primary circuit 6a, thereby further increasing the cost and reducing the design flexibility of the power supply circuitry. In addition, the large inrush current through the switch 12 inevitably shortens the operational life of the switch 12. This is yet another disadvantage of the prior art circuit 5.
Furthermore, despite the presence of the impedance element 11 of FIGS. 1A and 1B for partially suppressing inrush current and the severe specifications of the switch 12 in this prior art circuit 5, a spontaneous inrush current sufficiently large to damage the switch 12 can still be generated occasionally when the switch 12 is closed. As such, yet another disadvantage of this prior art circuit 5 is that the reliability of the switch 12 and the power supply circuit 5 as a whole is lower than desirable.
FIG. 2 illustrates another prior art circuit 7 whereby a tactile switch 22 is located in the secondary circuit 8b to which the load is also coupled. A tactile switch 22 is operated by light touch or gentle pressing and is commonly used in electronic appliances such as home audio and video equipment. Unlike a toggle switch which physically make or break the circuit by means of a moving contact, a tactile switch 22 usually operates in conjunction with a micro-controller to implement power switching. As is well known in the art, the micro-controller detects pressing action on the tactile switch 22 and turns on or shuts down the secondary circuit 8b by controlling the power supply circuit 7 via a feedback signal.
By using a tactile switch 22 in the secondary circuit 8b to which the load is coupled, instead of having a switch in the primary circuit 8a, this prior art circuit 7 effectively turns off the load by entering a `standby mode`, in which power is no longer being consumed in the secondary circuit 8b, while circumventing the requirement of a switch with severe specifications to withstand a large inrush current in the primary circuit 8a. Nevertheless, the power supply circuit 7 is still consuming power because the AC power input 20 is not disconnected from the power supply controller 25 in the primary circuit 8a by operation of the tactile switch 22 in the secondary circuit 8b. Recently, however, an increasing number of consumer electronics applications require a power supply circuit which can achieve "zero watt" or "near zero watt" power consumption when the load is turned off. This prior art circuit 7 does not meet this minimal power consumption requirement because the primary circuit 8a remains on and continues to consume power through the circuit elements, in particular the power supply controller 25, when the circuit 7 is in standby mode. It is advantageous to supply a low cost power supply that also meets the "zero watt" or "near zero watt" power consumption requirement.
To summarize, there is a need for a power supply circuit which does not require a high current switch. Additionally, there is a need for a power supply circuit that does not consume excessive power when the load is turned off. In one prior art circuit 5, as illustrated in FIGS. 1A and 1B, a large inrush current dictates that the switch 12 be of severe specifications. As is explained above, these stringent requirements of the switch 12 necessarily compromises the space and cost considerations of the prior art circuit 5. Further, the reliability of the circuit 5 is still unsatisfactory because of the potentially damaging effect of a large inrush current on the switch 12 despite such countermeasures as increasing input impedance and using a severe specification switch 12. In another prior art circuit 7, as illustrated by FIG. 2, the tactile switch 22 can power down the secondary circuit 8b but not the primary circuit 8a. Unfortunately, this means that power consumption through the power supply controller 25 and other circuit elements in the primary circuit 8a continues even when the secondary circuit 8b is turned off. Thus, these two prior art circuits 5 and 7 fail to address the full set of needs that are required of a power supply circuit for use in modern electrical and electronic appliances.